1. Field of the Invention
The present invention relates to knitting machines and more particularly concerns an electromechanical needle selection mechanism for a knitting machine having a needle bed accomodating a plurality of needles and associated jacks.
In such knitting machines, each needle is controlled by a pushing member or jack for selectively lifting the needle to perform a knitting operation. The selector members are controlled by an electromagnet and have the task of placing the jack as required in an active position, from which position a cam lifts the jack, and the jack lifts the needle until a needle butt engages a cam which then lifts the needle to knit.
2. Description of the Prior Art
Some of the known mechanisms comprise two electromagnets and a permanent magnet attached to the selector member to return this to its rest position. With this arrangement, the selector member moves from one electromagnet to the other when the polarity their is inverted. These systems require a double number of electromagnets, with all the disadvantages that this entails.
A further known device uses two counteracting springs associated with each needle. One of these springs tends to hold the associated jack constantly in the selection position. At each feed, the second of said springs engages a fixed cam which cocks it with centrifugal movement to move it into the proximity of a fixed selection electromagnet. If the needle is not to be selected, this electromagnet repels said second spring which then engages a second cam which moves it in a centripetal direction. During this movement, this second spring presses the jack against the action of said first spring and thus separates it from a lifting cam. Otherwise, the electromagnet holds this second spring until it engages with the second cam, thereby preventing it from operating against the first spring, so that said jack is raised by the lifting cam.
With this device, the electromagnet works only on contact with the spring, which is an advantage over the constructions wherein the electromagnet must attract the spring itself.
However, this construction has from many other disadvantages. In the first place, it only allows selection at a single level per feed. Each needle has to be provided with two counteracting springs. Consequently, there is twice the number of springs as needles.
The second spring works under disadvantageous conditions as a result of having to overcome the bias of the first spring.
It is known that in this construction, the means allowing a reduction of the power of the electromagnet have concomitant disadvantages which are substantially as troublesome as those they allow to be overcome, so that the proposed solution does not provide any real technical progress.
Moreover, certain knitting machines use selection at several levels by disposing several stacked selectors for each feed, in order to increase the time available for performing selection, giving the possibility of accelerating the speed of relative movement of the selectors and the needle bed carrying the needles and, consequently, of increasing production.
Therefore, it is important that the selection devices used should be neither too large nor too expensive, while still providing complete operational reliability. The space occupied and the price of electromagnets are significant if one consider that a machine may have, for example, 48 feeds of 10 selectors each, making a total of 480 selectors. Thus, if each selector uses two electromagnets, as in one of the aforementioned solutions, 960 electromagnets would be needed.
Other devices provide for a sliding movement of the selector members for engagement with the corresponding electromagnet and they generally have the disadvantage of requiring very close tolerances in the limits of such movement. If the movement is insufficient, the selector member does not contact the electromagnet and this is not strong enough to attract the member, whereby the required selection does not take place. On the other hand, if the selector member contacts the electromagnet prior to its position of maximum recoil, friction and tension are produced causing operational deficiencies in the knitting machine's mechanical performance.